Once upon a time there was a mermaid who wanted to walk on two legs into the world. And so she did, and she lived with a human princess who gave her a home and holidays and loved her very much.
But the mermaid didn't want to be merely human. She wanted to return to her home in the sea, she wanted to be a mermaid again. No love was worth giving up her home.
But the human princess loved the mermaid so much, that it broke her heart when she found the note the mermaid had left. So the human princess found a sorcerer who turned her into a mermaid too. And she followed her beloved mermaid into the sea.
And they lived happily ever after.